(a) Technical Field
The present invention relates to an apparatus and a method for controlling a driving mode of a hybrid electric vehicle (HEV). More particularly, the present invention relates to an apparatus and a method for controlling a driving mode of a hybrid electric vehicle which release an HEV mode at an appropriate time before forcibly releasing an engine clutch even when an accelerator pedal is engaged while driving the vehicle on an uphill road in the HEV mode, thereby reducing a shock or jerk caused when the engine clutch is forcibly disengaged and improving riding comfort and drivability.
(b) Background Art
Generally, a hybrid vehicle is a type of vehicle driven using an internal combustion engine (ICE) which uses a fossil fuel and a motor which uses electric energy as driving sources and is a type of eco-friendly vehicle which uses both the energy of the fossil fuel and the electric energy while being driven, to reduce exhaust gas and improve fuel efficiency. As a power train of the hybrid vehicle, a transmission mounted electric device (TMED) type in which a motor for driving the vehicle is disposed within a transmission is well known. In the general TMED type, the transmission is disposed at an output side of the motor and an engine clutch is disposed between the engine and the motor and thus, when the engine clutch is coupled to the engine and the motor, power of the engine and the motor is transmitted to a driving wheel through the transmission.
FIG. 1 is a view illustrating a configuration of a power train of a hybrid vehicle according to the related art and illustrates a configuration of a TMED type power train in which a motor is attached on a transmission. As illustrated in FIG. 1, a configuration of a TMED type power train includes an engine 1 and a motor 3 disposed in series as driving sources for driving a vehicle, an engine clutch 2 disposed to selectively connect or disconnect power between the engine 1 and the motor 3, a transmission 4 which changes power of the engine 1 and the motor 3 to output the power to the driving wheel, and a hybrid starter and generator (“HSG”) 5 connected to the engine 1 to transmit the power.
The engine clutch 2 connects or disconnects power between two driving sources which drive the vehicle, that is, the engine 1 and the motor 3 through selective operation of coupling and releasing operation. As is well known in the related art, when the hybrid vehicle is driven in an electric vehicle (EV) mode, the engine clutch 2 is separated to drive the vehicle using the power of the motor 3 and when the hybrid vehicle is driven in a hybrid electric vehicle (HEV) mode, the engine clutch 2 is coupled to drive the vehicle using the power of the engine 1 and the motor 3.
While a vehicle brakes (e.g., the brake pedal is engaged) or coasts by inertia, the motor 3 operates as a generator to perform an energy recovery mode in which a battery 7 is charged. The battery 7 which is a power source (e.g., an electric power source) of the vehicle is connected to the motor of the vehicle, that is, the motor 3 and the HSG 5 via an inverter 6 to be charged or discharged. In particular, the inverter 6 is configured to convert a direct current of the battery 7 into a three phase alternative current (AC) to apply the current to the motor 3 and the HSG 5, thereby driving the motor 3 and the HSG 5.
The HSG 5 is a device configured to perform a combined function of a starting motor and a generator in the hybrid vehicle and is driven as a motor using battery power when an engine is started to transmit the power to the engine 1 using power transmission equipment, such as a belt and a pulley, and is configured to operate as a generator by a rotational force transmitted from the engine 1 when electricity is generated, to thus charge the battery 7. In the meantime, in an area where high power is required, such as a highway or an uphill road, the hybrid vehicle is driven in the HEV mode which uses engine power and motor power and uses the engine power even when a state of charge (SOC) of a high voltage main battery is insufficient to secure SOC balancing while maintaining an appropriate SOC.
The TMED type hybrid vehicle equipped with an automatic transmission (e.g., a six speed transmission) applies a high torque based on fuel efficiency and the state of charge (SOC) balance after starting an engine, which is different from a general gasoline vehicle. Specifically, when a system does not include a torque converter, a high torque should be applied at all times. In particular, due to a characteristic of the engine, precision for controlling a torque is reduced at a low speed (e.g., low revolutions per minute, RPM) and when a high torque is applied at a low speed, noise, vibration, and harshness (NVH) performance deteriorates.
When the accelerator pedal is engaged with a minimal pedal amount (e.g., opening degree, minimal pressure is exerted onto the pedal) on an uphill road with a high driving load to drive the vehicle using the engine power through the HEV mode, a louder booming noise may be output and a substantial vibration of the engine may be experienced compared to the manipulation of a greater pedal opening degree (e.g., when more pressure is exerted onto the pedal). However, since the accelerator pedal is continuously engaged, an engine off condition is not satisfied, and the HEV mode is maintained.
In other words, when pressure is continuously exerted onto the accelerator pedal in the uphill section, a driver continuously requires the power, and thus the vehicle is maintained to be driven in the HEV mode. In summary, when the vehicle is driven on the uphill road, the engine power is used to maintain an appropriate SOC of the high voltage main battery and secure the SOC balancing even though the vehicle is driven with a minimal accelerator pedal amount and when the driver maintains the manipulation state of the accelerator pedal in the driving mode, the HEV mode is not released.
However, when an engine speed and a motor speed are reduced together with the vehicle speed due to the uphill driving where the vehicle enters a speed (RPM) area at which the HEV mode may not be maintained, an engine clutch is forcibly disengaged to prevent engine stall. While the HEV mode driving is performed with a minimal accelerator pedal amount on the uphill road, the vehicle speed should be rapidly decreased compared to flat road driving or downhill driving.
The rapid reduction in the vehicle speed, that is, a wheel speed of the vehicle means that the engine speed and the motor speed are rapidly decreased. In particular, when the speed reaches the low speed area where the HEV mode is not maintained, the engine clutch is operated to be forcibly disengaged, instead of normal clutch releasing sequence in which the engine clutch is completely disengaged after hydraulic disengagement.
As described above, when engagement of the engine clutch is forcibly disengaged to a state in which a hydraulic pressure is applied to the engine clutch to couple the engine torque and the clutch without having a normal control process for releasing an HEV mode, a vibration phenomenon such as shock or jerk (e.g., instant sudden movement) is generated together with the vibration of a driving shaft, which may lower drivability.
In the related art, a control strategy which releases the HEV mode at an appropriate time before a forcible releasing condition of the engine clutch in an uphill driving, an HEV mode driving, and an accelerator manipulation maintaining state is absent, increasing concern regarding a shock and jerk on the uphill road which connects levels of parking lots or an uphill congestion section in the city.
The above information disclosed in this section is merely for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.